iw the Foetus of Vertehratcd AnimaU. 105 



smaller as it proceeds onwards ; and the parent trunk, very small 

 at the place where it gives off' the lateral branch, gradually in- 

 creases in size by the accession of the cross branches, till it is 

 joined again by the lateral trunk itself. As the leaflets of the 

 gills become larger, they are covered by very numerous capillary 

 vessels, formed by the looping out or subdivision of the cross 

 branches, in a manner somewhat analogous to the extension of 

 vessels in the external gills*. The three anterior pairs of branchial 

 vessels, on coming out of the gills, before uniting with one an- 

 other and with the posterior pair, to form the descending aorta, 

 give arterial branches to the head, neck, and anterior extremities. 



The rudiments of the lungs are to be found adhering to the 

 lower side of the oesophagus, at the time when the external gills 

 are fully formed (Fig. 16, A, / /'). As development proceeds, 

 the little dense masses constituting the lungs become hollow 

 (Figs. 13, /, and 16, B) ; they gradually expand, their parietes 

 becoming membranous ; they are filled with air, and their cavity 

 is divided into cells by transverse septa rising from their sides 

 (Fig. 14, / /). The lungs of the frog, like those of the salaman- 

 der, receive their vessels from the posterior or fourth branchial 

 arch (Fig. 14, jo), or that nearest the heart; but in the frog, 

 this artery traverses the gill before arriving at the lung ; it 

 becomes gradually larger as the lungs become developed and 

 the period of transformation approaches, till the foetal life is at 

 an end, when it carries a proportion of blood considerably greater 

 than the three anterior arches. 



As the gills shrink and become smaller, the minute vessels of 

 their leaflets first disappear, and then the cross branches from 

 which they arose are obliterated. The apertures or clefts be- 

 tween the branchial hoops are closed up, the leaflets are partly 

 absorbed, and the branchial hoops themselves are gradually 

 softened down and removed. The large lateral branches of the 

 branchial arteries also begin to carry less blood, and are at last 



• These two parallel vessels, it is obvious, represent the rudimentary state 

 of the branchial vessels of fishes ; the large lateral tributary branch corre- 

 sponding with the artery, and the main trunk with the vein. It appears not 

 improbable, that the vessels of the gills in fishes are subdivided in a similar 

 manner to that alluded to above ; though, at the same time, itraustbe|acknow. 

 ledged that the double vessel has not as yet been remarked in the foetal fish. 



